Most water pumps have a strainer to keep the big stuff that will damage the pump out. The only time I've seen anything in one has been in pop-ups that aren't maintained, and the stuff has been green (shudder). The strainer should be more than enough for onboard water, plus, a filter could cause enough of a restriction the pump flow can be reduced significantly.
Yep, Terry I should have said Strainer instead of Filter. Something to keep small chunks of rust or sand out of the pump. On my Newell it is seperate from the pump.
Thanks for clarifying.